1. FIELD OF THE INVENTION
The present invention relates to an automatic focus detection device for use in a camera for detecting a focus position of a photographic lens of the camera by radiating an auxiliary ray to an object.
2. DESCRIPTION OF THE PRIOR ART
In an automatic focusing control, when luminance of the object to be photographed is low, it is difficult to obtain a good automatic focus detection under an ambient light source. In order to improve this problem , there has been proposed to use an auxiliary ray of light emitted from an auxiliary light source to give a high luminance of the object so as to obtain a good result of the automatic focus detection easily. The automatic focusing device of the type mentioned above is called as an active type automatic focus detection device.
FIG. 1 is a schematic view for explaining the principle of the active type automatic focus detection device. The light beam reflected from the object passes through a photographic lens 71 and a semi transparent mirror disposed between the photographic lens 71 and a film surface S and is reflected by a total reflection mirror 73 coupled with the semi transparent mirror 72 and is focused on a light receiving unit 74 made of a plurality of light receiving elements. The defocus amount is calculated by detecting the relation of the positions of the focused images or the contrast of the images and the photographic lens is driven depending on said calculated defocus amount so as to control the focusing of the camera. In this case, the focus detection becomes impossible if the amount of the light beam reflected by the object is insufficient. Therefore, there has been proposed an automatic focus detection device of active type as shown in Japanese Patent Laid Opened No. 126023/1979, wherein there are provided an auxiliary light source 75 and a projection lens 76 for projecting a light beam from the auxiliary light source 75 onto the object and the focus detection can be executed by receiving the auxiliary rays of light projected to the object. The auxiliary light source 75 is desirably accommodated in the camera. But in practice, it is difficult to provide the light source 75 in the camera because such arrangement causes the camera body to be bulky. Therefore, the inventor of the present invention has proposed an automatic focus detection device, wherein the auxiliary light source is accommodated in a flash device but in this case the flash device should be always attached to the camera body.
Generally the wavelength of the auxiliary light for the purpose of the automatic focus detection (referred to as AF detection hereinafter) is approximately 700 nm, which is within the range of visible light for human eyes. However, the directivity of the auxiliary rays is made sharp for the purpose of focus detection so that in many cases the photographer can not confirm the light emission of the auxiliary light and it is undesirable to share the auxiliary light source with an indicator.
There is known another active type automatic focus detection device in which the auxiliary light source is modified. In Japanese Patent Laid Opened No.151507/1985, there is provided means for projecting non-spot rays which are weaker than spot rays to the object in a short distance so as not to injure the objected person's eyes by looking in at the strong spot rays beam from a short distance. In Japanese Patent Laid Opened No.193406/1984, there is provided a diffraction grating in the optical system of the projection rays beam and plural diffraction rays are projected to the plural objects so as to prevent the projection rays beam from passing through between the two objects for example in case the two objects are little far away, thereby obtaining a correct focus detecting. In the above-mentioned known arts, the auxiliary light sources are modified for the purpose of solving the disadvantages in the automatic focus detection of active type or inconvenient points in practical use. However, both prior arts mentioned above disclose only to use the light source for AF control and never suggest to share the light source for the AF control as another use.